In the semiconductor industry, even complex integrated circuits are produced by means of basic processes, wherein many of the processes are geared to producing local structures on a wafer or any other suitable support. To that end, it is conventional practice for part of the wafer to be covered with a structured photoresist (referred to e.g. as soft mask) or some other structured material (referred to e.g. as hard mask). These structured layers are conventionally produced photolithographically, i.e. by means of an exposure process in which a pattern of an exposure mask is imaged onto a light-sensitive layer, e.g. a photoresist layer, or onto some other suitable layer.
Exposure processes have been established for a long time, and there are a large number of variations, e.g. with regard to the section-by-section exposure of a wafer by means of a so-called stepper, with regard to the wavelength used (e.g. UV light or deep UV light with a wavelength of less than 300 nm), etc.
The photoresist layer is correspondingly developed after the partial exposure. In this case, the photoresist layer is partly removed, such that this can be used as a structured masking layer. By means of this masking layer, a layer to be treated (e.g. a wafer to be treated or a layer to be treated on a wafer) arranged below the masking layer can be locally altered, e.g. doped, removed (e.g. by means of etching), etc. Furthermore, it is also possible for further layers to be grown locally by means of the masking layer, e.g. by means of a so-called lift-off process or the like.
The exposure masks used for the exposure of a light-sensitive layer usually consist of a transparent carrier, on the surface of which a pattern (i.e. at least one exposure structure) of non-transparent or less transparent regions, for example composed of chromium, is produced. During the exposure of a light-sensitive layer, the pattern of the exposure mask is imaged onto the light-sensitive layer by means of an exposure apparatus. To that end, the light-sensitive layer is positioned with respect to the imaging optical unit of the exposure apparatus (that is to say that a predefined exposure geometry is set) in such a way that the pattern of the exposure mask is imaged onto the light-sensitive layer in as focused a manner as possible, that is to say that the light-sensitive layer is positioned in accordance with the image distance b, which results from the object distance g and the focal length f of the optical unit. The relationship is essentially described by means of the imaging equation:
      1    f    =            1      b        +                  1        g            .      
In order to ensure an optimum quality of the exposure, it is conventional practice to use a test wafer for reference exposures, wherein different test regions of the test wafer are exposed using different exposure geometries. By way of example, the distance between the test wafer and the imaging optical unit (for example a lens arrangement or a mirror arrangement) of the exposure apparatus is varied during the exposure of the test regions of the test wafer. By means of the evaluation of the test wafer, it is then possible to determine the optimum exposure geometry for the exposure of further wafers. However, this is time- and cost-intensive. Furthermore, after the exposure of a plurality of wafers, demonstrating whether the respective wafers were exposed correctly, e.g. at the optimum focus, is possible only with comparative difficulty.